This invention relates to a solid electrolyte battery of the thermal type. In a more specific aspect, this invention concerns itself with a lightweight, thermally activated, solid state, electrochemical power supply which utilizes a solid alkali metal tetrachloroaluminate electrolyte in combination with a transition metal chloride containing cathode.
The present interest in the development of lightweight electrical energy supplies for a variety of military and commercial applications has created a considerable research effort in an attempt to provide even more reliable and efficient thermally activated, solid state electrolyte batteries. Generally, the term "thermal battery" is understood to mean a battery which is activated by heating until its electrolyte melts, thereby becoming a good electrical conductor and a solvent in which the appropriate energy-producing chemical reactions can occur. The concept of thermally activating a power source provides a system which is characterized by having a virtually unlimited shelf life coupled with an extended operational life. This is accomplished by the fact that the battery is only activated when a source of power is required. Such batteries have proved to be especially useful where long life and very small currents are required, such as heart pacemaker units and permanently powered computer memory boards. These batteries are light in weight and of minimum size.
In furthering the research effort referred to above, considerable emphasis has been placed on developing power sources which utilize an alkali metal tetrachloroaluminate composition as a solid electrolyte. Such batteries are generally in pelletized form and provide a lightweight, maintenance free, high energy density power source with a relatively long shelf-life. Unfortunately, the tetrachloroaluminate solid electrolyte batteries heretofore fabricated do not possess as high a current carrying capacity and voltage as is desired for many applications.
With the present invention, however, it has been found that the use of a cupric chloride, ferric chloride, or molybdenum chloride cathode material in conjunction with an alkali metal tetrachloroaluminate electrolyte and a lithium alloy anode, provides an electrochemical power source with an unexpectedly greater current carrying capacity and higher voltage than previously exhibited by the solid state tetrachloroaluminate battery described in U.S. Pat. No. 4,066,824, issued Jan. 3, 1978 to Rao et al. The battery of this invention is also somewhat related in concept to the aluminum-chlorine thermal battery described in U.S. Pat. No. 4,064,327, issued Dec. 20, 1977 to King et al; and the tetrachloroaluminate battery described in U.S. Pat. No. 4,207,388 issued June 10, 1980 to Erbacher et al.
The battery or electrochemical power of this invention, however, differs fundamentally from the prior art in that it is not a thermal battery in accordance with the accepted definition of the term "thermal battery". Although the battery of this invention is structurally similar to a thermal battery and is thermally activated, it differs in that the electrolyte does not melt during operation, but relies instead on a significant solid state electrical conductivity. To thermally "activate" the battery means to bring it up to whatever temperature results in an acceptable value of electrolyte electrical conductivity. The battery must be externally maintained at that minimum temperature since the battery is not thermally self-sustaining.
The use of a transition metal chloride as a cathode material in conjunction with a tetrachloroaluminate solid electrolyte results in substantial improvement in open circuit voltages and current carrying capacities over the voltages and current capacities exhibited by previously known chloroaluminate batteries. This unexpected improved improvement is achieved because the metal chloride is used in a positive oxidation state rather than relying on alkali metal intercalation for the discharge reaction as is done in solid electrolyte cells which use chalcogenide containing cathodes, such as those disclosed by Rao et al, supra.